Piezo actuator

ABSTRACT

A piezoelectric actuator in which a piezoelectric element is present for subjecting an actuating element to a tensile or compressive stress. A sleeve is secured to a foot part, to which the piezoelectric element is secured and by way of which the piezoelectric element is kept centered in a housing under mechanical prestressing, and this sleeve surrounds the piezoelectric element in such a way as to stabilize it mechanically, at least in partial regions that are electrically insulated from the piezoelectric element. Between the sleeve and the piezoelectric element, a heat-conducting elastomer is placed, and the sleeve comprises a heat-conducting material that is deformable within predetermined limits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 01/01329 filed onApr. 5, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an improved piezoelectric actuator foractuating a mechanical component such as a valve or the like.

2. Description of the Prior Art

It is generally known that by using the so-called piezoelectric effect,a piezoelectric element can be constructed from a material having asuitable crystalline structure. When an external electrical voltage isapplied, a mechanical reaction of the piezoelectric element ensues,which as a function of the crystalline structure and the regions ofcontact with the electrical voltage exerts a pressure or tension in apredeterminable direction.

From German Patent Disclosure DE 196 50 900, for instance, apiezoelectric actuator is known which is suitable particularly foractuating control valves or injection valves in motor vehicles. To thatend, it comprises layers, stacked on one another in the manner of alaminate, of piezoelectric material with metal or electricallyconductive layers serving as electrodes located between them. Theactuator body thus formed, on each of whose end faces a respective headplate toward the valve, with an axially protruding valve tappet, and anopposed foot plate are attached, is inserted into a retaining bore of avalve housing. On the face end, the actuator body is prestressed in theaxial direction, between the head plate and the foot plate, by a springelement.

As mentioned at the outset, such piezoelectric multilayer actuators,when they are subjected to a pulsating electrical voltage on theirelectrode layers, execute similarly pulsating strokes, changing thespacing between their two face ends. The piezoelectric multilayeractuator must be supported in the steel housing of the injection valvein such a way that short circuits cannot occur, which means that acentered installation with a defined spacing from the outer wall must beassured, and there must be no risk of short circuits between theactuator body and the outer wall.

In operation of such piezoelectric multilayer actuators, heat occurs inthe actuator body, and if destruction and impairment of the function ofthe multilayer actuator is to be avoided, this heat must be dissipatedto the outside, or in this case to the valve housing of the injectionvalve or control valve. In the known multilayer actuator mentionedabove, the spring sleeve absorbs the heat generated by the actuator bodyand carries it to the outside and simultaneously assures the centering,required for installation, of the actuator body in the valve housing. Itis intrinsically known from German Patent DE 197 15 488 C1 that anactuator body be surrounded with a polymer envelope, but this envelopeleaves an air gap open between itself and the inner wall of the valvehousing. The result is the disadvantage that the dissipation of heatthrough the air gap to the valve housing is made more difficult.

SUMMARY OF THE INVENTION

In the piezoelectric actuator of the invention, it is assumed that thereis a piezoelectric element which is suitable for subjecting an actuatingelement to a tensile or compressive stress and which is provided with afoot part to which the piezoelectric element is secured and by way ofwhich the piezoelectric element is kept centered in a housing withmechanical prestressing. Advantageously, a sleeve is secured to the footpart and surrounds the piezoelectric element in such as way as tostabilize it mechanically, at least in partial regions that areelectrically insulated from the piezoelectric element.

It is especially advantageous if a heat-conducting elastomer is placedbetween the sleeve and the piezoelectric element, and the sleevecomprises a heat-conducting material that is deformable withinpredetermined limits. This component group containing the piezoelectricelement and the sleeve on the foot part can be press-fitted in a simpleway into a retaining bore of the housing, resulting in good thermalconduction from the piezoelectric multilayer actuator, for instance to avalve housing. The foot part can also have an encompassing groove, intowhich a likewise encompassing groove can be latched on the insidediameter of the sleeve.

With the sleeve, the piezoelectric actuator has a surface that is notvulnerable, for instance to chemical factors, and it can be installedsimply, and an actuator module of this kind is also protected againstharmful impairments during shipping and in assembly. The piezoelectricactuator cast in the sleeve is both centered and secured againstrotation by the elastomer, since the flow of force is not only via thefoot part but additionally via the elastomer to the sleeve that is alsojoined to the foot part.

Compared to a piezoelectric actuator without a sleeve and sheathed withonly a heat-conducting elastomer, the production process is also moreeconomical, since the requisite care taken of the casting molds isunnecessary. Assembling the piezoelectric actuator of the invention isalso simpler, because injuries to the sheathing and the attendant outputof heat-conductive particles, with consequent soiling of the retainingbore, cannot occur, as they can in a piezoelectric actuator that issheathed only with elastomer.

The sheath of the invention, which is as a rule thin-walled and whichcomprises the heat-conducting and deformable material, such as copper,steel or a plastic, is solidly joined to the foot part of thepiezoelectric element, and to improve the centering, it can have aslightly increasing outer diameter toward the top. A metal sleeve, forinstance, can be produced economically in the form of a rolled componentof stamped sheet metal. A plastic sleeve, for instance in the form of aninjection-molded part, is then allowed to touch the piezoelectricelement and can thus lend the actuator module greater stability evenwithout elastomer sheathing.

In a further embodiment, it is advantageous if by means of centeringrings additionally placed in the sleeve and preferably having a hollowprofile, the piezoelectric element can be centered even better in thesleeve, while at the same time space is created for the compensation ofthermal and press-fitting expansion.

By means of an oblique shoulder in the retaining bore, a calibratingpress-fitting of the sleeve can also be made possible in a simple way,thus assuring a close contact of the sleeve with the retaining bore, asa result of which very good heat conduction radially outward takesplace.

In another embodiment of the piezoelectric actuator of the invention,the sleeve is perforated or slotted, so that the heat-conductingelastomer can flow through this sleeve and directly produces the heatconduction between the piezoelectric element and the housing having theretaining bore. The casting of the piezoelectric element can be doneeither in the housing or in a re-usable or lost-type mold.

These and other characteristics of preferred refinements of theinvention can be learned from the description and the drawings; theindividual characteristics may each individually or in the form ofsubcombinations with each other be realized in the embodiment of theinvention and in other fields and can represent both advantageous andintrinsically patentable embodiments for which patent protection is hereclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the piezoelectric actuator of the invention,for example for a fuel injection system in a motor vehicle, will now beexplained in conjunction with the drawings, in which:

FIG. 1 is a section through a piezoelectric actuator, with a sleeve andan elastomer layer;

FIGS. 2-5 are sectional views of the foot part with differentpossibilities for securing the foot part to the piezoelectric element;

FIG. 6, a section through a piezoelectric actuator with a sleeve havingrecesses in the form of holes;

FIG. 7, a detail of the sleeve of FIG. 6;

FIG. 8, a section through a piezoelectric actuator with a sleeve withrecesses in the form of oblong slots; and

FIG. 9, a detail of the foot part, with a further possibility ofsecuring it to the piezoelectric element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a piezoelectric actuator 1 is shown in the form of apiezoelectric multilayer actuator, which has a piezoelectric element 2that in a manner known per se is constructed from piezoelectric sheetsof a quartz material with a suitable crystalline structure, so that byusing the so-called piezoelectric effect, when an external electricalvoltage is applied to electrodes, not shown here, a mechanical reactionof the piezoelectric actuator 1 ensues, in the form of an axial motion.

On its fixed end, the piezoelectric actuator 1 is centered and solidlyjoined to a foot part 3 of steel or ceramic. On the foot part 3, adeformable and highly heat-conductive sleeve is put in place and joinedto the foot part 3, for instance by welding. The sleeve 4 has a lesserdiameter in the region of the foot part 3, since the sleeve 4, in theprocess of being press-fitted into a retaining bore 5, is unable toyield radially in this region because of the foot part 3. The sleeve 4is made for instance of copper, steel or a plastic, and it can be eitheras long as the entire piezoelectric element 2 or can be shorter, so asto reinforce only the connection between the piezoelectric element 2 andthe foot part 3.

The piezoelectric element 2 is cast in the sleeve 4 with aheat-conducting elastomer 6. Via a taper 7 in the retaining bore 5, forinstance in a valve housing 8, this component combination ispress-fitted into the retaining bore 5; the sleeve 4 has only a slightoversize compared to the retaining bore 5 in the valve housing 8, sothat production tolerances in the housing 8 and the sleeve 4 can becompensated for, and so that good heat conduction is established betweenthe housing 8 and the sleeve 4.

Located on the moving end of the piezoelectric actuator is a head partwith a thrust bolt 9, which is either solidly joined to thepiezoelectric element 2 or merely rests on it and which braces thepiezoelectric actuator 1 via a spring 10 in such a way that thepiezoelectric actuator is subjected during operation only to compressivestresses. The nonpositive engagement, or flow of force, extends here viaa fixation shim 11 and a securing ring 12 into the valve housing 8 andback into the piezoelectric element 2 again via the foot part 3.

Between the elastomer 6 and the sleeve 4, on one side, and the head part9 on the other, a gap 13 is formed, which serves to compensate forthermal expansion and deformation in the press-fitting operation. Thecentering of the piezoelectric actuator 1 is then effected via thesleeve 4, and not via the foot part 3.

The sleeve 4 can be secured to the foot part 3, for instance beingsoldered, welded, or glued, or secured by the methods described below inconjunction with FIGS. 2-4. In FIG. 2, gnurling 14 is applied to thecircumference of the foot part 3, and the sleeve 4 is press-fitted ontothe gnurling with positive engagement. In the exemplary embodiment ofFIG. 3, the sleeve 4 is shown as a deep-drawn part, with a cap 15 onwhich the foot part 3 is mounted. In this case, the flow of forcetravels during operation from the piezoelectric element 2 via the cap 15of the sleeve 4 into the valve housing 8. As a further variant form ofsecuring, FIG. 4 shows an encompassing groove 16 in the foot part 3,into which the sleeve 4 is crimped.

In an exemplary embodiment of FIG. 5, the piezoelectric element 2 isprovided with a centering ring 17, which rests on the foot part 3. Thecentering ring 17 is embodied in such a way that it traps an air volumeand can be compressed by the elastomer 6 upon thermal expansion of theelastomer.

In FIG. 6, an exemplary embodiment of the piezoelectric actuator 1 isshown which has a sleeve 20 with recesses, in this case holes 21 oradditional vertical slots. The electrical terminals 22 for thepiezoelectric element 2 protrude from the bottom side of the foot part 3and are passed through the foot part 3. Once again, the piezoelectricelement 2, sleeve 20 and foot part 3 are mounted in the housing 8 andare surrounded by a heat-conducting elastomer 6, which either surroundsthese components in a casting mold before assembly or is not cast intothe housing 8 until in the assembled state. This elastomer 6 serves todissipate the lost energy, converted into heat, from the piezoelectricelement 2 during operation to the housing 8. FIG. 7 shows a modifiedform of the sleeve 20 with a vertical slot 23.

In FIG. 8, a variant of the embodiment of the sleeve 20 with oblongslots 24 can be seen. Here there is also a recess 25 for the electricalterminals 22 of the piezoelectric element 2.

In the exemplary embodiment of FIG. 9, a further possible connection ofthe foot part 3 to the sleeve 4 or 20 is shown. The piezoelectricelement 2 is placed in centered fashion on the foot part 3, and in thefoot part 3, in a shoulder 26 serving to receive the sleeve 4 or 20, agroove 27 is made into which an encompassing bead 28 is latched in onthe inside diameter of the sleeve 4 or 20 and thus joins the twocomponents to one another.

The foregoing relates to preferred exemplary embodiment of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A piezoelectric actuator, comprising at least onepiezoelectric element for subjecting an actuating element to a tensileor compressive stress, said piezoelectric element having a moving endand a fixed end opposite the moving end, a foot part (3) to which thefixed end of the piezoelectric element (2) is secured and by way ofwhich the piezoelectric element (2) is kept centered in a housing (8)under a mechanical prestressing, a sleeve (4, 20) secured to the footpart (3) and surrounding the piezoelectric element (2) In such a way asto stabilize the sleeve (4, 20) mechanically, at least in regions thatare electrically insulated from the piezoelectric element, the housing(8) having a retaining bore (5) for receiving the piezoelectric actuatorand into which the sleeve (4, 20) together with the piezoelectricelement (2) are firmly press-fitted optionally via an oblique shoulder(7) on the periphery of the retaining bore (5).
 2. The piezoelectricactuator of claim 1, further comprising a heat-conducting elastomer (6)placed between the sleeve (4, 20) and the piezoelectric element (2). 3.The piezoelectric actuator of claim 1, wherein the sleeve (4, 20)comprises a heat-conducting material that is deformable withinpredetermined limits.
 4. The piezoelectric actuator of claim 2, whereinthe sleeve (4, 20) comprises a heat-conducting material that isdeformable within predetermined limits.
 5. The piezoelectric actuator ofclaim 3, wherein the sleeve (4, 20) is of metal.
 6. The piezoelectricactuator of claim 4, wherein the sleeve (4, 20) is of metal.
 7. Thepiezoelectric actuator of claim 3, wherein the sleeve (4, 20) is ofplastic.
 8. The piezoelectric actuator of claim 4, wherein the sleeve(4, 20) is of plastic.
 9. The piezoelectric actuator of claim 1, whereinthe sleeve (20) has recesses (21, 23, 24, 25) in its wall.
 10. Thepiezoelectric actuator of claim 2, wherein the sleeve (20) has recesses(21, 23, 24, 25) in its wall.
 11. The piezoelectric actuator of claim 3,wherein the sleeve (20) has recesses (21, 23, 24, 25) in its wall. 12.The piezoelectric actuator of claim 5, wherein the sleeve (20) hasrecesses (21, 23, 24, 25) in its wall.
 13. The piezoelectric actuator ofclaim 7, wherein the sleeve (20) has recesses (21, 23, 24, 25) in itswall.
 14. The piezoelectric actuator of claim 1, wherein the foot part(3) has an encompassing groove (27), into which a likewise encompassingbead (28) can be latched on the inside diameter of the sleeve (4, 20).15. The piezoelectric actuator of claim 2, wherein the foot part (3) hasan encompassing groove (27), into which a likewise encompassing bead(28) can be latched on the inside diameter of the sleeve (4, 20). 16.The piezoelectric actuator of claim 5, wherein the foot part (3) has anencompassing groove (27), into which a likewise encompassing bead (28)can be latched on the inside diameter of the sleeve (4, 20).
 17. Thepiezoelectric actuator of claim 1, further comprising a centering ring(17) between the piezoelectric element (2) and the sleeve (4, 20). 18.The piezoelectric actuator of claim 2, further comprising a centeringring (17) between the piezoelectric element (2) and the sleeve (4, 20).19. The piezoelectric actuator of claim 9, wherein, after thepiezoelectric element (2) with the sleeve (20) has been inserted into aretaining bore (5) in the housing (8) for the piezoelectric actuator(1), the remaining interstices in the retaining bore (5) are filled atleast partially with a heat-conducting elastomer (6).
 20. Thepiezoelectric actuator of claim 14, wherein, after the piezoelectricelement (2) with the sleeve (20) has been inserted into a retaining bore(5) in the housing (8) for the piezoelectric actuator (1), the remaininginterstices in the retaining bore (5) are filled at least partially witha heat-conducting elastomer (6).